1. Field of the Invention
This invention relates to oil and gas well production technology. More particularly, it relates to the in situ treatment of fluids produced by an artificial lift oil well to inhibit the formation of scale inside and outside of production tubing, pumps, valves, and the like and to reduce the amount of solids that enter the pump.
2. Description of the Related Art
A typical oil well produces not only oil, but also gas and water, often in significant quantity. The fluids often transport solids, such as sand, as well as other potentially damaging fluids and gases, from the reservoir into the production tubing and casing, and up the production tubing to the surface. Equipment on the surface may be used to separate these production components. The oil is recovered; the gas, depending on its composition, may be filtered, treated and piped to a collection facility or flared off; the water may be re-injected into another formation or, in the case of offshore production platforms, treated to prevent environmental contamination and then discharged overboard; and the solids are separated and disposed of.
The oil and water produced by oil and gas wells often contains significant quantities of dissolved minerals. Frequently, the water is saturated with these minerals—i.e., the water contains the maximum concentration of the dissolved minerals possible at a given temperature and pressure. Changes in temperature and/or pressure which occur as the fluid is pumped from the production zone through the well to the treatment equipment on the surface can cause the minerals to come out of solution (“precipitate”) and become deposited on the interior and exterior surfaces of the production tubing, pumps, valves, chokes and other equipment. The deposit is known as “scale” and it can significantly reduce the diameter and hence the capacity of production tubing. In extreme cases, the pipe or tubing can become completely obstructed, shutting down production. Even in less severe cases, where the fluid is not saturated, scale can build up on the interior and exterior of any exposed surface.
Certain dissolved minerals in water are known as “hardness ions” —divalent cations that include calcium (Ca+2), magnesium (Mg+2) and ferrous (Fe+1) ions. Hardness ions develop from dissolved minerals, bicarbonate, carbonate, sulfate and chloride. Heating water containing bicarbonate salts can cause the precipitation of a calcium carbonate solid. Raising the pH can allow the Mg+2 and Fe+2 ions to precipitate as Fe(OH)2 and Mg(OH)2. Excess sodium carbonate can precipitate Ca+2 as CaCO3.
Precipitation is the formation of an insoluble material in a solution. Precipitation may occur by a chemical reaction of two or more ions in solution or by changing the temperature of a saturated solution. There are many examples of this important phenomenon in drilling fluids. Precipitation occurs in the reaction between calcium cations and carbonate anions to form insoluble calcium carbonate: Ca+2+CO3−2→CaCO3.
Scale is a mineral salt deposit or coating formed on the surface of metal, rock or other material. Scale may be caused by a precipitation resulting from a chemical reaction with the surface on which it forms, precipitation caused by chemical reactions, a change in pressure or temperature, or a change in the composition of a solution. The term “scale” is also applied to a corrosion product. Typical scales are calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, iron sulfide, iron oxides, iron carbonate, the various silicates and phosphates and oxides, or any of a number of compounds insoluble or slightly soluble in water.
Scale may be deposited on wellbore tubulars, down hole equipment, and related components as the saturation of produced water is affected by changing temperature and pressure conditions in the production conduit. In severe conditions, scale creates a significant restriction, or even a plug, in the production tubing. Scale build-up in the artificial lift pump can lead to failure of the pump due to blocked flow passages and broken shafts. Scale removal is a common well-intervention operation. A wide range of mechanical, chemical and scale inhibitor treatment options are available to effect scale removal.
Scale can also occur in tubing, the gravel pack, the perforations or the formation itself. Scale deposition occurs when the solution equilibrium of the water is disturbed by pressure and temperature changes, dissolved gases or incompatibility between mixing waters. Scale deposits are the most common and most troublesome damage problems in the oil field and can occur in both production and injection wells.
All waters used in well operations can be potential sources of scale, including water used in waterflood operations and filtrate from completion, workover or treating fluids. Therefore, reduction of scale deposition is directly related to reducing the amount of bad water that is produced.
Carbonate scale is usually granular and sometimes very porous. A carbonate scale can be easily identified by dropping it in a solution of hydrochloric acid where bubbles of carbon dioxide will be observed effervescing from the surface of the scale. Sulphate scales are harder and more dense. A sulphate deposit is brittle and does not effervesce when dropped in acid. Silica scales resemble porcelain—they are very brittle, not soluble in acid, but dissolve slowly in alkali.
Scale removal is a common well-intervention operation involving a wide variety of mechanical scale-inhibitor treatments and chemical options. Mechanical removal may be done by means of a pig or by abrasive jetting that cuts scale but leaves the tubing intact. Scale-inhibition treatments involve squeezing a chemical inhibitor into a water-producing zone for subsequent commingling with produced fluids, preventing further scale precipitation. Chemical removal is performed with different solvents according to the type of scale:                Carbonate scales such as calcium carbonate or calcite [CaCO3] can be readily dissolved with hydrochloric acid [HCl] at temperatures less than 250° F. [121° C.].        Sulfate scales such as gypsum [CaSO4.2H2O] or anhydrite [CaSO4] can be readily dissolved using ethylenediamine tetraacetic acid (EDTA). The dissolution of barytine [BaSO4] or strontianite [SrSO4] is much more difficult.        Chloride scales such as sodium chloride [NaCl] are easily dissolved with fresh water or weak acidic solutions, including HCl or acetic acid.        Iron scales such as iron sulfide [FeS] or iron oxide [Fe2O3] can be dissolved using HCl with sequestering or reducing agents to avoid precipitation of by-products, for example iron hydroxides and elemental sulfur.        Silica scales such as crystallized deposits of chalcedony or amorphous opal normally associated with steamflood projects can be dissolved with hydrofluoric acid [HF].        
Calcium scales such as calcium sulfate, calcium carbonate and calcium oxalate are insoluble in water. However, all three are soluble in a Sodium Bisulfate acid solution. Calcium scale can be removed with an acid wash using a 5-15% solution of Sodium Bisulfate (SBS). SBS can also be used during a shut down to remove scale by re-circulating it throughout areas of the process where needed. The concentration of SBS solutions and the re-circulation time depend on the amount of scale that needs to be removed. SBS can be a substitute for sulfamic acid in calcium scale removal situations.
Zinc sulfide (ZnS) is another one of the oil field scales that plagues production. Although it does not seem to be common, according to field experience and published literature, it causes a significant flow/production problem when it does occur, just as all other scales adversely affect wells. Other scales, such as barium sulfate and strontium sulfate, also cause production problems but are much harder than ZnS.
Although chemical solvents have been used on these harder scales, the results are often disappointing. While mechanical scale removal has been used successfully on barium and strontium sulfate scales with excellent success, it had not been used on ZnS scale. It was conceivable that the softer scale may not respond to the same process that removed harder scales.
In certain cases, scale may be an environmental or health hazard. The State of Louisiana, Department of Environmental Quality has issued a notification concerning a potential health hazard associated with handling pipe used in oil and gas production that may be contaminated with radioactive scale from naturally-occurring radioactive materials (NORM). The concern is the possible inhalation and/or ingestion of scale particles contaminated with radium-226 and possibly other radioactive material that may become airborne during welding, cutting or reaming pipe that contains radioactive scale. The State of Louisiana is using the term Technologically Enhanced Natural Radiation (TENR) for this material that is a subset of the NORM group.
An inhibitor is a chemical agent added to a fluid system to retard or prevent an undesirable reaction that occurs within the fluid or with the materials present in the surrounding environment. A range of inhibitors is commonly used in the production and servicing of oil and gas wells, such as corrosion inhibitors used in acidizing treatments to prevent damage to wellbore components and inhibitors used during production to control the effect of hydrogen sulfide [H2S]
A scale inhibitor is a chemical agent added to a fluid system to retard or prevent an undesirable reaction that occurs within the fluid or with the materials present in the surrounding environment. A range of inhibitors is commonly used in the production and servicing of oil and gas wells, such as corrosion inhibitors used in acidizing treatments to prevent damage to wellbore components and inhibitors used during production to control the effect of hydrogen sulfide [H2S]
A sequestering agent (or chelation agent) is a chemical whose molecular structure can envelop and hold a certain type of ion in a stable and soluble complex. Divalent cations, such as hardness ions, form stable and soluble complex structures with several types of sequestering chemicals. When held inside the complex, the ions have a limited ability to react with other ions, clays or polymers. Ethylenediamine tetraacetic acid (EDTA) is a well-known sequestering agent for the hardness ions, such as Ca+2, and is the reagent solution used in the hardness test protocol published by API. Polyphosphates can also sequester hardness ions. Sequestering is not the same as precipitation because sequestering does not form a solid. For calcium carbonate deposits, glycolic and citric acids and ammonium salts and blends incorporating EDTA are used as chelants.
A scale-inhibitor squeeze is a type of inhibition treatment used to control or prevent scale deposition. In a scale-inhibitor squeeze, the inhibitor is pumped into a water-producing zone. The inhibitor is attached to the formation matrix by chemical adsorption or by temperature-activated precipitation and returns with the produced fluid at sufficiently high concentrations to avoid scale precipitation. Some chemicals used in scale-inhibitor squeezes are phosphonated carboxylic acids or various polymers.
Some scale-inhibitor systems integrate scale inhibitors and fracture treatments into one step, which guarantees that the entire well is treated with scale inhibitor. In this type of treatment, a high-efficiency scale inhibitor is pumped into the matrix surrounding the fracture face during leakoff. It adsorbs to the matrix during pumping until the fracture begins to produce water. As water passes through the inhibitor-adsorbed zone, it dissolves sufficient inhibitor to prevent scale deposition. The inhibitor is better placed than in a conventional scale-inhibitor squeeze, which reduces the re-treatment cost and improves production.
Some well treatment systems continuously inject the treating chemical in the well using a metering pump. The chemicals are either injected below the pump using a capillary line or injected into the well annulus. When chemicals are injected into the well annulus the chemicals build up in the well bore until the pump pulls them down the wellbore and into the pump intake.
Due to the time that it takes for the chemicals to reach the pump, changes in chemical mix or injection rates are very slow to affect the fluids entering the pump. If the pump intake is above the electric motor in an Electric Submersible Pump, ESP installation, the chemicals do not protect the motor or the casing below the pump intake.
In capillary injection systems, the location of the chemical injection can be determined when the system is installed by terminating the capillary tube below the pump intake/motor combination in an ESP completion. The capillary injection tube provides continuous treatment of the fluids and the time delay for adjustments to the blend of chemicals and/or treatment rate can be minimized.
Sand produced with the fluids can cause damage to pumping systems. Abrasion resistant pumps with engineered ceramic bearings and coated flow passages have been developed to improve pump life in wells that produce sand, but sand will eventually wear out even these special sand-tolerant pumps.
One practice for removing sand from the fluid is by installing a liquid and sand separator between the casing perforations and the pump intake. These systems deposit the separated sand into the well's rat hole or into tubing hung from the bottom of the separator as a trap. Wilson discloses a means for removal of sand separated with a downhole sand separator in U.S. Pat. No. 6,216,788.
Gravel packing is a sand-control method used to prevent the production of formation sand. It involves the placement of selected gravel across the production interval to prevent the production of formation fines or sand. Any gap or interruption in the pack coverage may permit undesirable sand or fines to enter the producing system.
In gravel pack operations, a steel screen is placed in the wellbore and the surrounding annulus is then packed with prepared gravel of a specific size that is designed to prevent the passage of formation sand. The primary objective is to stabilize the formation while causing minimal impairment to well productivity.
Wire-wrapped screen is one type of screen used in sand control applications to support the gravel pack. The profiled wire is wrapped and welded in place on a perforated liner. Wire-wrapped screen is available in a range of sizes and specifications, including outside diameter, material type and the geometry and dimension of the screen slots. The space between each wire wrap must be small enough to retain the gravel placed behind the screen, yet minimize any restriction of production.
A sand filter as described by Stanley in U.S. Pat. No. 4,977,958 is used to filter the sand out of the fluid prior to entering the pump intake. This style of intake filter has been installed in numerous wells and is effective for removal of solids, but once the filter is full of sand, fluid flow through the filter is restricted and a large pressure drop occurs. As the pressure drop increases, the rate of sand accumulation increases causing the rate of pressure drop to increase until eventually the fluid flow across the filter ceases. When fluid flow to the pump ceases, the pump will cavitate and eventually fail.